Evaluation of anatomical pelvic parameters between normal, healthy men and women using three-dimensional computed tomography: a cross-sectional study of sex-specific and age-specific differences

Sagittal spinal balance and standing posture are affected by pelvic morphology, especially pelvic incidence (PI) [1, 2]. Abnormal sagittal alignment of the spine may lead to difficulty in maintaining proper balance, i.e., “hip-spine syndrome” [3]. Sacral slope (SS) and pelvic tilt (PT) [1, 2] are commonly used pelvic parameters; they are considered positional parameters because they are affected by the position of the subject when they are examined with two-dimensional (2D) radiographic images. Conversely, PI is considered an anatomical parameter because it does not vary regardless of the subject’s position [1, 2].

The importance of PI was previously reported for regulating sagittal balance, which leads to optimal lordosis of the lumbar spine and thoracic kyphosis [4]. Moreover, several investigations have reported significant correlations between PI and 2D-PT and 2D-SS [5‐8].

Pelvic parameters, even PI, measured by using 2D radiographic images are influenced by the rotation of the pelvis [9, 10]. Many studies have reported the disadvantage of the 2D measurement with regard to accuracy; measurement error was between 3 and 6° [11, 12].

Vrtovec et al. [13] originally described that the three-dimensional (3D) measurement by using reconstructed images from computed tomography are not affected by the projection plane, rotation, and/or lateral tilt of the pelvis; additionally, it is not difficult to identify the hip center because of overlap of the pelvis, image contrast, and soft tissue artifacts [14‐16]. Measurements of PI are not always suitable in all patients, especially those with osteoarthritis of the hip joint whose femoral head is nonspherical, subluxed, or dislocated.

Moreover, 2D-PT and 2D-SS change depending on the subject’s position; therefore, the relationship of these angles as anatomical angles is unclear.

If the bone model of the pelvis is adjusted to the same reference plane of the pelvis, PI, PT, and SS are all considered as anatomical parameters. There are no reports on the relationship between PI, anatomical-PT, and anatomical-SS.

The purposes of this study were to evaluate pelvic morphological parameters using a 3D measurement obtained by computed tomography and compare them between normal, healthy men and women and younger and older subjects. We also validated this 3D measurement by using intraobserver and interobserver reliabilities. We hypothesized that the relationship between PI, anatomical-PT, and anatomical-SS would be different between men and women because the morphological feature of the pelvis is different between men and women [17, 18]. This 3D measurement may have a high reliability.

Details of the 108 participants are as follows: 55 men (age 49.3 ± 30.1 [range 19–79] years, body height 166.0 ± 11.4 [range 153–179] cm, body weight 63.3 ± 16.6 [range 42–79] kg, body mass index 22.9 ± 4.5 [range 15.6–29.3] kg/m²) and 53 women (age 49.1 ± 29.6 [range 18–79] years, body height 151.9 ± 12.1 [range 135–164] cm, body weight 52.4 ± 14.6 [range 43–73] kg, body mass index 22.7 ± 5.6 [range 17.0–29.6] kg/m²). The group of subjects younger than 50 years consisted of 26 men (28.1 ± 21.6 [range 19–49] years) and 28 women (31.2 ± 22.0 [range 18–46] years), whereas the group of subjects aged 50 years or older consisted of 29 men (64.5 ± 20.8 [range 53–79] years) and 25 women (63.8 ± 21.9 [range 53–79] years).

There was no significant difference in PI, anatomical-PT, anatomical-SS, the distance in the Y_p coordinate, distance in the Z_p coordinate, and total distance between men and women (Table 1). There was a strong correlation between PI and anatomical-SS in both men and women (R = 0.790 and 0.715, respectively) (Tables 2 and 3).

With regard to age-specific differences, the values of anatomical-PT were lower (p values, men 0.019, women 0.048) and values of anatomical-SS were greater among older men and women (p values, men 0.039, women 0.012), whereas the value of PI was similar between younger and older male and female subjects (p values, men 0.960, women 0.360) (Tables 4). Moreover, the distance in the Y_p coordinate was significantly lower among older male and female subjects (p values, men 0.009, women 0.046), whereas there were no significant differences in the distance in the Z_p coordinate (p values, men 0.362, women 0.196) and total distance (p value, men 0.173, women 0.128) (Table 4).

Regarding validation, the results showed that intraobserver mean absolute differences were 1.8 mm for total distance and 1.8° for PI and anatomical-SS, and the minimal correlation coefficient was 0.890 for anatomical-SS (Table 5). However, the interobserver mean absolute difference was slightly larger than the intraobserver mean absolute difference (maximum mean absolute differences were 2.1 mm for total distance and 2.2° for anatomical-SS), and the minimal correlation coefficient was 0.876 for anatomical-SS (Table 5).

With regard to the post hoc analysis, power values were 0.825 according to the t test and 0.945 in the correlation analysis of men and women, 0.572 according to the t test and 0.745 in the correlation analysis of men aged < 50 years and men aged ≥ 50 years, and 0.559 according to the t test and 0.730 in the correlation analysis of women aged < 50 years and women aged ≥ 50 years, respectively.

In our study, we found that pelvic parameters such as PI, anatomical-PT, and anatomical-SS were similar between men and women. Therefore, the relative location of the center of the S1 superior end plate and hip axis was similar between male and female subjects; it was well known that there are several differences in the morphology and/or contour of the pelvis between men and women [17, 18]. Consequently, pelvic parameters are valid to use without any distinction of sex. Moreover, the distance in the Y_p coordinate, distance in the Z_p coordinate, and total distance were also similar between men and women, whereas body height was approximately 1.1 times larger in men than in women in this study. We preliminarily adjusted the total distance of men to conform with the body height of women; the adjusted total distance of men was calculated as 97.9 using the following formula: 107.0 × (151.9/166.0), and the difference of the total distance between men and women increased over 10 mm. Therefore, these distances were considered as relatively larger in women than in men with consideration of the difference in body height. This morphological difference may be due to the width of the birth canal in women.

There was a strong correlation between PI and anatomical-SS in both male and female subjects and younger and older groups. This novel concept may be useful to estimate PI from anatomical-SS because the measurements of PI are not always suitable in all patients, especially those with osteoarthritis of the hip joint whose femoral head is not spherical or whose femoral head is subluxed or dislocated. Based on our results, the formula could be calculated to estimate PI from anatomical-SS as follows: PI = 0.79 × anatomical-SS + 17.76.

With regard to the values of PI, they were similar between male and female subjects and younger and older subjects. For normal subjects, it is generally accepted that PI increases during childhood and then remains unchanged throughout adolescence and adulthood [6, 24]. Our results were similar to those in previous studies [6, 24]. However, several studies reported that PI was influenced by age and significantly larger in late adulthood [25, 26]. The differences between our study and these studies are unclear. These previous studies and our study were cross-sectional, not longitudinal; therefore, further analysis should be done.

We speculated that the sacrum; the center of the S1 superior end plate was tilted anteriorly and translated anteriorly simultaneously with aging; thus, anatomical-PT was lower and anatomical-SS was larger in older subjects than in younger patients in both sexes.

In 2D measurements, 2D-PT and 2D-SS are considered as positional parameters, and only PI is a morphological parameter. Many previous studies have measured PI using 2D sagittal radiographs in standing position [4, 11, 12, 27]. They reported that the measurement error was between 3 and 6° [11, 12]. Regarding the measurement of PI using the 3D method, few studies have been reported, but Vrtovec et al. [13] reported that mean PI was 47° in 370 normal subjects, mean absolute difference was 2.5 ± 2.3°, and the correlation coefficient was 0.946. We also obtained a lower measurement error using this 3D method than using a previously reported 2D measurement [9, 12]. However, it is difficult to examine the computed tomography scans of all patients for several reasons such as the high cost and radiation exposure; therefore, further investigation is required to validate these novel 3D measurements even in the previously reported 2D method. Our results were considered to have high reliability, similar to those of Vrtovec et al. (Table 1).

The current study has several limitations. First, only a few subjects were enrolled. Second, this study was a cross-sectional study; therefore, the differences between younger and older subjects may not always be true due to long-term changes. Further investigation of the relationship between these parameters may be required to explore the detailed changes of these parameters as they are altered by aging.

We found no differences in anatomical references, such as PI, anatomical-SS, and anatomical-PT, between men and women. However, there was a strong correlation between PI and anatomical-SS among younger and older subjects in both sexes. This novel measurement concept may be useful to estimate PI from anatomical-SS because the measurements of PI are not always suitable in all patients, especially those with osteoarthritis of the hip joint whose femoral head is not spherical or whose femoral head is subluxed or dislocated.